Optical communication is a known technique by which data signals can be transmitted from a transmitter to a receiver using optical fiber. Typically, an optical transmitter converts an electrical signal into an optical signal, which forms a carrier wave. The carrier wave is modulated with a modulation signal (i.e. the data), and is then transmitted along the optical fiber to a receiver, which converts the optical signal back into an electrical signal and recovers the transmitted data.
A single optical fiber can be used to transmit a large number of individual data streams, or channels, by transmitting radiation having a range of wavelengths (for example broadband radiation) through the optical fiber, and using a wavelength-division multiplexing (WDM) system to divide the radiation into discrete wavelengths or wavebands. A separate data stream can then be transmitted within each waveband.
In order to recover the data transmitted in one data stream (i.e. in one waveband) within an optical fiber, it is necessary to route the data stream out of the optical fiber to a receiver. One known method for achieving this is through the use of an optical add-drop multiplexer (OADM), which routes one data stream out of the optical fiber, and routes another data stream into the optical fiber to make use of the empty waveband.
An example of a system 100 using a known optical add-drop multiplexer (OADM) 102 is shown schematically in FIG. 1. In the exemplary system 100, wavelength-division multiplexing is used to divide the radiation passing through an optical fiber 104 into four discrete wavebands λ1, λ2, λ3 and λ4. Each waveband is capable of carrying a single data stream.
An optical fiber grating which, in this case is a fiber Bragg grating (FBG) 106, is formed within the optical fiber 104. A fiber Bragg grating is an optical instrument which can be configured to reflect radiation at a particular wavelength (or in a particular waveband) and to transmit radiation at all other wavelengths. In the example shown in FIG. 1, the FBG 106 is configured to reflect radiation having wavelength within the waveband λ4, and transmit all other radiation, including the radiation having wavelengths within the wavebands λ1, λ2 and λ3. Therefore, while the data streams being transmitted in the wavebands λ1, λ2 and λ3 are able to travel along the optical fiber 104 through the FBG 106, the data stream being transmitted in the waveband λ4 is reflected by the FBG back along the optical fiber in the direction from which it came.
The reflected radiation is fed into a first optical circulator 108 which routes the reflected radiation out of the optical fiber 104 to a receiver (not shown). The radiation that is routed out of the optical fiber 104 by the OADM 102 is known as a “dropped” path. Since the data stream that was in the waveband λ4 has been removed from the optical fiber 104, it is possible to add a new data stream to be transmitted in the waveband λ4 (i.e. an “added path”). This is achieved by modulating a carrier wave in the waveband λ4, and feeding it into the optical fiber 104 after the FBG 106. In order to provide a carrier wave at the desired wavelength (i.e. in waveband λ4), the known system 100 uses a laser (not shown). Radiation at the desired wavelength λ4 from the laser is fed into the optical fiber 104, and a second optical circulator 110 routes the radiation along the optical fiber in the original direction of transmission.
A system such as the exemplary system 100 might be positioned at each node in an optical fiber communication network. Such a communication network might be installed in an aircraft, where optical fiber networks are preferred to electric cable networks due to their resilience to electromagnetic interference compared to electrical cables, and due to the fact that optical fibers are generally less heavy than shielded electrical cables. However, for each node in the network to be able to transmit data, each node would require a laser source which itself is very power intensive, and may require an active cooling system. Installing a laser source at each node in a network on an aircraft can negate the benefits provided by the lighter cables.